Engine swap wiring

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ThatGuy
Posts: 479
Joined: Wed Feb 08, 2012 2:41 am
Car(s): 1986 2.0 laser
Location: Surrey

Engine swap wiring

Post by ThatGuy » Sat Jul 20, 2019 3:07 pm

Just need to get the wiring done on my capri then fingers crossed she'll run (doubtful I know)
So I've done an engine swap for an mx5 engine and the intention was to use the mx5 loom for the engine and the standard capri loom for all the electrics like headlights and that kind of stuff.
Currently I've got the mx5 loom in the car connected to the engine which I plan to wire to the battery and through the ignition, and I've removed all the wiring for the old pinto as it's redundant.
My question is how do I go about wiring the rest of the capri loom so it has power for everything, as isn't it usually fed from the other side of the alternator?
Any help would be amazing as I'm fucking shit at wiring and my brain hurts just thinking about it :)
Ratty as fuck

Not_Anumber
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Donator
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Joined: Sun Jan 15, 2017 3:54 pm
Car(s): 2.8 Injection 1983, early 5 speed. Slowly restoring a 1979 MGB + a 1964 Honda 250 twin. Previously owned 2 Capris.

Re: Engine swap wiring

Post by Not_Anumber » Mon Jul 22, 2019 1:40 pm

Here goes: the first thing to know is that some alternators have 2 wires, others have 3.

You happen to have a car wired for a 2 wire alternator but you now have an alternator with a 3 wire system. Its easy to work around this:

For all negative earth cars the main (heaviest) alternator output cable runs to the positive terminal on the battery and to the main cable to the starter motor and also provides the main unswitched power to the fusebox & the ignition switch. All alternators have this. its simple, unswitched positive.

In a 2 wire alternator the other wire goes to the charge/ ignition light on the dashboard. This causes a very small current drain on the alternator which allows it to work. You may have read that Capri's have charging problems when Fuse number 7 blows and that's because the charge light is wired via this fuse. If the bulb or the fuse goes the alternator wont do its job. (In 3 wire alternator systems this charge light circuit is usually less critical)

In a 3 wire alternator the 3rd wire has to be connected to a switched live. This can be from any circuit that is live when the ignition is on - same as the wipers or the stereo for instance. Some modern cars actually run this from a special ECU instead that can cleverly switch off the alternator when the battery is fully charged. Not sure if the MX5 is wired to do this but Id think it more likely given the age that the MX5 would just use a bug standard switched live- something you can easily provide from one of the Capri's own circuits.

This guy replaced a pinto with a mazda engine same as you so the answers will all be applicable https://www.locostbuilders.co.uk/forum/ ... tid=200512

ThatGuy
Posts: 479
Joined: Wed Feb 08, 2012 2:41 am
Car(s): 1986 2.0 laser
Location: Surrey

Re: Engine swap wiring

Post by ThatGuy » Mon Jul 22, 2019 7:50 pm

Dude that's awesome, thanks for taking the time to explain that to me, really helped me out :)
Ratty as fuck

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